1. Field of the Invention
The present invention relates to the field of electronics. More specifically, the present method and apparatus relate to methods and systems for controlling a permanent magnet motor having a speed sensor with poor or coarse resolution.
2. Description of the Related Art
Permanent magnet motors are generally regarded today as an interesting solution for a wide range of inverter-fed variable-speed drives. Advantages of these motors in comparison to state of the art asynchronous motors include lower losses and higher torque density.
The motor control industry as a whole is a strong and aggressive sector. In order for members of the industry to remain competitive, they must not only reduce costs imposed by governments and power plant lobbies, but also answer to power consumption reduction and EMI radiation reduction issues. One consequence of these constraining factors is the need of enhanced control strategies for permanent magnet motors.
Present permanent magnet motor control strategies typically require high-resolution sensors. A permanent magnet motor has available a number of various kinds of sensors that can be used, such as optical encoders which are subject to contamination, and resolvers which require a great deal of circuitry. Such sensors are expensive and are not very reliable. Accordingly, there is a need for a control strategy that provides excellent motor speed control without the use of high-resolution sensors.
In the disclosed embodiment, the present method and apparatus alleviate the drawbacks described above with respect to known control strategies for permanent magnet motors by employing algorithms that estimate motor speed. The present method and apparatus utilize a series of nested loops to give the power signals a uniform ground reference with associated signal stability and signal noise reduction.
The control strategy of the present method and apparatus is useful for high performance equipment, including but not limited to fuel cells, compressors, blowers and the like. High performance equipment requires very accurate speed control with limited information and good dynamic response. Other motor control strategies do not allow such good performance with limited information. By providing good performance with limited information, cost is driven down, e.g., a very robust and very expensive sensor is no longer needed.
In contrast to the prior art, the control strategy of the present method and apparatus allows the substitution of an estimated motor voltage for an actual measured speed of a permanent magnet motor. By utilizing an estimated motor voltage, a less expensive speed sensor having only coarse resolution may be utilized. Additionally, the estimated voltage is more reliable than the measured speed.
Three nested control loops determine the estimated voltage. The outermost control loop is a speed regulator. Generally, an external speed command xcfx89* (for rotational speed) is provided to the control system, e.g., from a fuel cell controller. This is compared to a speed measurement xcfx89, which is very slow, creating a speed error. The speed regulator functions to drive this error to zero. This loop must operate at a low repetition rate due to the limitations of the available speed measurement.
The output of the speed regulator is an outer loop voltage command V*, which is compared against a motor voltage V measured in the second control loop, a voltage regulator loop. The voltage regulator loop operates at a higher rate than the speed regulator loop because the feedback quantity is a higher bandwidth signal than the speed measurement. The voltage regulator likewise attempts to drive the error to zero, producing a current command I*.
The last loop, a current regulator loop, takes the current command I* and measures the physical current and calculates the desired instantaneous motor voltage required to synthesize the commanded current. This instantaneous voltage is integrated or averaged and fed back as the measured motor voltage (inner loop voltage command feedback signal) that is used by the voltage regulator.
Compensation for changing bus voltage is also provided in the form of a feed forward voltage function. The feed forward voltage function is provided to ensure a steady state and calculate the terminal voltage of the motor based upon the speed command. A pulse width modulator and inverter is provided which is comprised of a set of switches that will, over some period of time, average the value of the voltage that is applied. For example, suppose the system commands 50 volts. If the bus voltage is 200, a 25% duty cycle is commanded. With 25% of 200 and 75% of zero, the average over time is 50 volts.
The system advantageously allows the use of a low resolution speed measurement in high performance applications. The voltage regulator loop provides a very good substitute for the motor speed measurement as it has relatively high bandwidth compared to the available physical measurement. The voltage regulator loop also has constant dynamics, unlike a speed sensor that typically has quantization difficulty at high speeds and delay problems at low speeds.
The general beneficial effects described above apply generally to each of the exemplary descriptions and characterizations of the devices and mechanisms disclosed herein. The specific structures through which these benefits are delivered will be described in detail herein below.